Process monitoring method for monitoring the joining of at least two workpieces by means of a self-piercing riveting device having joining tongs, and device having a control unit designed to carry out such a method

ABSTRACT

A process monitoring method for monitoring the joining of at least two workpieces by means of a self-piercing riveting device having a joining gun including a) a total sheet metal thickness, a rivet length and a bending constant of the joining gun are detected for a plurality of joining operations for each operation before joining; b) a punch path for a certain force, an end force, a path end value for the end force, a setting path of the rivet, and a rivet head end position are detected for the plurality of joining operations; c) a force/path window for the joining operations is determined; and d) a determined force/path window of one additional joining operation is compared to the force/path window of the joining operations determined in step c), and the additional joining operation is monitored so that any joining-related defects are identified.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No. PCT/EP2020/069822 filed Jul. 14, 2020, which designated the United States, and claims the benefit under 35 USC § 119(a)-(d) of German Application No. 10 2019 005 075.4 filed Jul. 19, 2019, the entireties of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a process monitoring method for monitoring the joining of at least two workpieces by means of a self-piercing riveting device having joining tongs, and a device having a control unit designed to carry out such a method.

BACKGROUND OF THE INVENTION

Self-piercing riveting methods serve to join at least two workpieces, wherein a rivet is pressed into the at least two workpieces by means of a punch or a punching tool, wherein it is ensured by a correspondingly shaped counter-holder, which cooperates with the punching tool, that the rivet or the workpieces deform in a specific manner in order to produce a frictional and positive connection between the workpieces.

There are known from the prior art self-piercing riveting methods in which quality control is carried out by means of an envelope of the force/displacement profile. When components of brittle material are joined by means of a self-piercing riveting method, faults, for example, cracks, can occur as a result of the exertion of the force. The occurrence of such a fault leads to certain short-time fluctuations in the force that is exerted on the punch and the components. It is thereby possible to check whether the force profile moves within specific limit values of the envelope. A deviation from the envelope leads to the installation being stopped, which makes it necessary to carry out a manual check and to discharge affected components. Not every deviation from the envelope means that the quality of the resulting components is unsatisfactory, and for this reason the components are examined and it is determined which components are actually affected. Generally, only a small proportion of the components is affected. The envelope is, therefore, learned on the basis of defined production lots. However, this procedure is complex since it must be carried out afresh for each type of component and a large number of parameters must be set.

Likewise known are methods wherein, in a self-piercing riveting device, the punching force and the gradient thereof in respect of the pressing-in displacement of the punch are determined during setting of the rivet. If a fault occurs in the riveted joint, there is a fluctuation in the gradient profile. The gradient of the force is thereby determined during specific time and/or displacement ranges during pressing in of the rivet. Fluctuations of the gradient of the punching force are generally comparatively small, so that such fluctuations are not detected within the context of monitoring of the force profile within specific limits.

German Laid-Open Specification DE 10 2016 214 943 A1 discloses a method for joining at least two components by means of a self-piercing riveting apparatus, wherein the at least two components are arranged between a punch and a counter-holder, wherein a rivet arranged between the punch and a component of the at least two components that faces the punch is pressed into the at least two components in that a force is applied to the punch, and wherein, during pressing in of the rivet, a gradient of the force is determined at least temporarily. A manufacturing device having such a self-piercing riveting apparatus is further disclosed.

However, it is a disadvantage of the methods known from the prior art that, when a limit value is applied to a force or to a gradient of the force which is necessary to press the rivet into the two components to be joined, joined parts which do not have any faults at all are rejected.

SUMMARY OF THE INVENTION

Accordingly, the object underlying the present invention is to provide a process monitoring method for monitoring the joining of at least two workpieces by means of a self-piercing riveting device having joining tongs, and a device having a control unit designed to carry out such a method, wherein the mentioned disadvantages do not occur and wherein, in particular, more accurate quality control is possible.

The object is achieved, in particular, in that there is provided a process monitoring method for monitoring the joining of at least two workpieces by means of a self-piercing riveting device having joining tongs, wherein the at least two workpieces are arranged between a punch and a counter-holder, and wherein a rivet arranged between the punch and a workpiece facing the punch is pressed by means of the punch into the at least two workpieces in that a force F in the direction towards the counter-holder is applied to the punch. The method is characterized in that a) a total thickness, a rivet length, and a bending constant of the joining tongs are recorded for a plurality of joining operations before joining, b) punch displacement at a specific force, an end force, a displacement end value at the end force, a setting displacement of the rivet, and a rivet head end position are recorded for the plurality of joining operations during and/or after joining, c) in dependence on the parameters recorded in step a) and step b), an absolute force/displacement window and/or a relative force/displacement end window of the plurality of joining operations is/are determined, and d) a determined absolute force/displacement window and/or a relative force/displacement end window of at least one further joining operation is/are compared with the absolute force/displacement window and/or the relative force/displacement end window determined in step c) of the plurality of joining operations, wherein the at least one further joining operation is monitored so that a fault in joining is identified.

Preferably, the at least two workpieces have a total thickness, in particular, a total sheet thickness.

Preferably, by application of the process monitoring method, the quality-determining regions are deterministically determined.

A fault is understood as being, in particular, a crack.

A quality-determining region is understood as being in particular a region of an absolute force/displacement window and/or a region of a relative force/displacement end window which is of importance for the quality of a joining operation and/or of joined at least two workpieces, in particular, in which a fault is identifiable.

Preferably, the workpiece is formed of a metal or a metal alloy.

Preferably, the joining of the at least two workpieces yields a component.

A component is understood as being, in particular, a part of a vehicle, in particular, a body component, preferably of a passenger car, truck, bus, motorhome, construction vehicle, commercial vehicle, or of a rail vehicle, boat, ship or aircraft.

Preferably, the component is a coated component, in particular, a painted or galvanized component.

Preferably, the process monitoring is carried out automatically.

The process monitoring method according to the present invention for monitoring the joining of at least two workpieces by means of a self-piercing riveting device having joining tongs has advantages compared to the prior art. Advantageously, improved quality control is ensured. Advantageously, the number of components incorrectly identified as faulty is reduced. Advantageously, the availability of components is increased, in particular, installation stoppages are reduced. Advantageously, costs are reduced and more rapid production is made possible. Advantageously, monitoring of the entire joining operation is not required. Advantageously, the learning outlay on commissioning of the installation and during series operation is reduced. Advantageously, specially trained staff are not required.

According to a further development of the present invention, it is provided that relevant quality-determining regions of the joining operation are determined, wherein the process monitoring is reduced to the relevant quality-determining regions.

Preferably, the quality-determining regions of an envelope are defined and the process monitoring is reduced to quality-determining regions of the envelope.

According to a further development of the present invention, it is provided that the relevant quality-determining regions are determined time-dependently and/or displacement-dependently.

Preferably, in step c), a force/displacement profile and/or a force/time profile, in particular, a gradient of a force/displacement profile and/or of a force/time profile, is/are recorded. Preferably, a gradient is determined from the force/displacement profile and/or from the force/time profile. It is thus ensured that faulty pressing of the rivet into the two workpieces to be joined is identified, since such faulty joining is in most cases associated with a jerky pressing-in movement. Such a jerky pressing-in movement is visible in a force/displacement profile as a steep rise or fall.

According to a further development of the present invention, it is provided that, in step d), a fault in joining is identified above a specific threshold value of a difference between the determined absolute force/displacement window and the relative force/displacement end window of the further joining operation and the determined absolute force/displacement window and/or the relative force/displacement end window of the plurality of joining operations.

A threshold value is understood as being, in particular, a specific difference between a determined absolute force/displacement window and a relative force/displacement end window of a further joining operation and a determined absolute force/displacement window and/or a relative force/displacement end window of the plurality of joining operations. Above the threshold value a fault in joining is identified while below the threshold value no fault in joining is identified. The threshold value can be determined or specified in dependence on the requirements.

According to a further development of the present invention, it is provided that the threshold value is determined by comparison with at least one standard component and/or by learning on the basis of a plurality of joining operations.

According to a further development of the present invention, it is provided that the parameters in step a) and in step b) are determined in at least one specific time and/or displacement range during pressing in of the punch, preferably in at least one time and/or displacement range in which, based on experience, a fault is to be expected.

According to a further development of the present invention, it is provided that a gradient is not used for the process monitoring, in particular for identifying a fault in joining, in particular, a gradient of the force during pressing in of the rivet is not used.

According to a further development of the present invention, it is provided that the recorded parameters in step a) and b) for determining the absolute force/displacement window and/or the relative force/displacement end window are weighted differently.

According to a further development of the present invention, it is provided that at least one further parameter, in particular, a force/displacement profile and/or a force/time profile of the joining operation, is used for the process monitoring.

The object is also achieved in that there is provided a device having a control unit designed to carry out a method according to the present invention, in particular, according to one of the exemplary embodiments described above, wherein the device further has a self-piercing riveting device having joining tongs, a punch and a counter-holder, wherein a force F in the direction towards the counter-holder can be applied to the punch so that a rivet can be pressed into at least two workpieces arranged between the punch and the counter-holder, wherein the device has sensors for recording the parameters, and at least one evaluation unit for determining an absolute force/displacement window and a relative force/displacement end window of a plurality of joining operations and comparing it with a determined absolute force/displacement window and/or a relative force/displacement end window of at least one further joining operation. In particular, the advantages already explained above in connection with the process monitoring method for monitoring the joining of at least two workpieces by means of a self-piercing riveting device having joining tongs are thereby obtained for the device.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be explained in greater detail hereinbelow with reference to the drawing. The single FIGURE shows, in a flow diagram, a schematic representation of an exemplary embodiment of a process monitoring method for monitoring the joining of at least two workpieces by means of a self-piercing riveting device having joining tongs.

DETAILED DESCRIPTION OF THE INVENTION

The FIGURE shows, in a flow diagram, a schematic representation of an exemplary embodiment of a process monitoring method for monitoring the joining of at least two workpieces by means of a self-piercing riveting device having joining tongs. In the process monitoring method for monitoring the joining of at least two workpieces by means of a self-piercing riveting device having joining tongs, the at least two workpieces are arranged between a punch and a counter-holder, wherein a rivet arranged between the punch and a workpiece facing the punch is pressed by means of the punch into the at least two workpieces in that a force F in the direction towards the counter-holder is applied to the punch. In step a), a total thickness, a rivet length, and a bending constant of the joining tongs are recorded for a plurality of joining operations before joining. In step b), a punch displacement at a specific force, an end force, a displacement end value at the end force, a setting displacement of the rivet, and a rivet head end position are recorded for the plurality of joining operations during and/or after joining. In step c), in dependence on the parameters recorded in step a) and step b), an absolute force/displacement window and/or a relative force/displacement end window of the plurality of joining operations is/are determined. In step d), a determined absolute force/displacement window and/or a relative force/displacement end window of at least one further joining operation is/are compared with the absolute force/displacement window and/or the relative force/displacement end window determined in step c) of the plurality of joining operations, wherein the at least one further joining operation is monitored so that a fault in joining is identified.

Improved quality control is thereby ensured. The number of components incorrectly identified as faulty is reduced. Advantageously, costs are reduced and more rapid production is made possible.

In an embodiment of the present invention, relevant quality-determining regions of the joining operation are determined, wherein the process monitoring is reduced to the relevant quality-determining regions.

In a further embodiment of the present invention, the relevant quality-determining regions are determined time-dependently and/or displacement-dependently.

In a further embodiment of the present invention, in step d) a fault in joining is identified above a specific threshold value of a difference between the determined absolute force/displacement window and the relative force/displacement end window of the further joining operation and the determined absolute force/displacement window and/or the relative force/displacement end window of the plurality of joining operations.

In a further embodiment of the present invention, the threshold value is determined by comparison with at least one standard component and/or by learning on the basis of a plurality of joining operations.

In a further embodiment of the present invention, the parameters in step a) and in step b) are determined in at least one specific time and/or displacement range during pressing in of the punch, preferably in at least one time and/or displacement range in which, based on experience, a fault is to be expected.

In a further embodiment of the present invention, a gradient is not used for the process monitoring, in particular for identifying a fault in joining.

In a further embodiment of the present invention, the recorded parameters in step a) and b) for determining the absolute force/displacement window and/or the relative force/displacement end window are weighted differently.

In a further embodiment of the present invention, at least one further parameter, in particular, a force/displacement profile and/or a force/time profile of the joining operation, is used for the process monitoring.

The method is carried out, in particular, in a device having a control unit which is designed to carry out the method. The device has a self-piercing riveting device having joining tongs, a punch and a counter-holder, wherein a force F in the direction towards the counter-holder can be applied to the punch so that a rivet can be pressed into at least two workpieces arranged between the punch and the counter-holder, wherein the device has sensors for recording the parameters, and at least one evaluation unit for determining an absolute force/displacement window and a relative force/displacement end window of a plurality of joining operations and comparing it with a determined absolute force/displacement window and/or a relative force/displacement end window of at least one further joining operation. 

1. A process monitoring method for monitoring the joining of at least two workpieces by means of a self-piercing riveting device having joining tongs, wherein the at least two workpieces are arranged between a punch and a counter-holder, and wherein a rivet arranged between the punch and a workpiece facing the punch is pressed by means of the punch into the at least two workpieces when a force F in the direction towards the counter-holder is applied to the punch, wherein a) a total thickness, a rivet length, and a bending constant of the joining tongs are recorded for a plurality of joining operations before joining, b) a punch displacement at a specific force, an end force, a displacement end value at the end force, a setting displacement of the rivet, and a rivet head end position are recorded for the plurality of joining operations during and/or after joining, c) in dependence on the parameters recorded in step a) and step b), an absolute force/displacement window and/or a relative force/displacement end window of the plurality of joining operations is/are determined, and d) a determined absolute force/displacement window and/or a relative force/displacement end window of at least one further joining operation is/are compared with the absolute force/displacement window and/or the relative force/displacement end window determined in step c) of the plurality of joining operations, wherein the at least one further joining operation is monitored so that a fault in joining is identified.
 2. The method as claimed in claim 1, wherein relevant quality-determining regions of the joining operation are determined, wherein the process monitoring is reduced to the relevant quality-determining regions.
 3. The method as claimed in claim 2, wherein the relevant quality-determining regions are determined time-dependently and/or displacement-dependently.
 4. The method as claimed in claim 1, wherein, in step d), a fault in joining is identified above a specific threshold value of a difference between the determined absolute force/displacement window and the relative force/displacement end window of the further joining operation and the determined absolute force/displacement window and/or the relative force/displacement end window of the plurality of joining operations.
 5. The method as claimed in claim 4, wherein the threshold value is determined by comparison with at least one standard component and/or by learning on the basis of a plurality of joining operations.
 6. The method as claimed in claim 1, wherein the parameters in step a) and in step b) are determined in at least one specific time and/or displacement range during pressing in of the punch.
 7. The method as claimed in claim 1, wherein a gradient is not used for identifying a fault in joining.
 8. The method as claimed in claim 1, wherein the recorded parameters in step a) and b) for determining the absolute force/displacement window and/or the relative force/displacement end window are weighted differently.
 9. The method as claimed in claim 1, wherein at least one of a force/displacement profile and/or a force/time profile of the joining operation, is used for the process monitoring.
 10. A device having a control unit designed to carry out a method as claimed in claim 1, wherein the device further has a self-piercing riveting device having joining tongs, a punch and a counter-holder, wherein a force F in the direction towards the counter-holder can be applied to the punch so that a rivet can be pressed into at least two workpieces arranged between the punch and the counter-holder, wherein the device has sensors for recording the parameters, and at least one evaluation unit for determining an absolute force/displacement window and a relative force/displacement end window of a plurality of joining operations and comparing it with a determined absolute force/displacement window and/or a relative force/displacement end window of at least one further joining operation.
 11. The method as claimed in claim 1, wherein the parameters in step a) and in step b) determined in at least one time and/or displacement range in which, based on experience, a fault is to be expected. 